The invention relates to current interrupters of the vacuum-type for use in controlling fault currents associated with transmission lines in power distribution systems. In particular, the invention relates to the shape of the vacuum envelope employed in the current interrupter portion of a current limiting circuit.
Increases in electric power demand has led utility systems to use higher voltages in the transmission of power. Fault currents, due to ground shorts for example, can rapidly become enormous on high voltage power distribution lines and can cause serious equipment damage. Therefore, as transmission voltages rise, there is a continuing need in the electric power industry for improved current limiting devices capable of rapidly controlling fault currents.
Current limiters generally employ a current-suppressive impedance in parallel with some type of current interrupter. The interrupter opens under a fault condition and the current is diverted through the impedance, which limits the current to a safe level. A vacuum-type current interrupter generally comprises a pair of relatively movable electrodes in a vacuum envelope. The electrodes can be placed in electrical contact to provide a free path for current flow. Means are associated with the interrupter to separate the electrodes when a fault current is detected. When the electrodes separate, arcing occurs across the gap between the electrodes as soon as the last point of contact has been broken. Since the arc continues to carry substantially the full fault current it becomes necessary to extinguish the arc if the current is to be successfully diverted through the current-suppressive impedance.
In the past it has been possible in alternating current power systems to permit the interelectrode arc in an interrupter to burn until a normal current zero is reached, at which time the arc disappears. Reignition of the arc is prevented if the dielectric strength across the electrode gap is sufficient to withstand the subsequent transient voltages. In present high-voltage lines, fault currents can build up to such a high value that even a single current half-cycle can cause damage to the transmission system. Instead, it is necessary to immediately suppress the arc. One method of extinguishing the arc is by application of a transverse magnetic field across the interelectrode gap of the vacuum device. The magnetic field causes a space charge in front of the anode which in turn causes a large voltage drop across the interrupter. This high voltage can then be used to force the current into the parallel current-suppressive impedance.
Arcing which occurs in vacuum causes the release of a cloud of metallic vapor containing conductive ions of electrode material. This metallic vapor forms a metallic deposit upon any surface it reaches. After repeated interruptions, this deposit can build into a continuous current path which can seriously degrade interrupter performance. For example, if the arc is driven by the magnetic field into the wall of the envelope when it is coated with metallic arc deposit, an arcing path will likely arise from one electrode to the wall and then back to the other electrode. This can prevent arc extinction because the magnetic field cannot divert an arc carrying current in a direction parallel to the lines of magnetic force. A magnetic field creates a Hall electric field which diverts an arc in the J X B direction and, unless the arc cuts across the lines of magnetic force, there is no resultant diverting force. When arcing initiates between the electrodes and the envelope wall, the arc tends to align itself with the magnetic field lines. If this happens, the arc is unaffected by the magnetic field and will continue to burn. It is therefore highly undesirable to have arcing proceed between the electrodes and the envelope wall.
It is preferable to have the wall of the vacuum envelope spaced far from the electrodes so as to discourage arcing between the electrodes and the wall. Most commonly this is done by increasing the diameter of the envelope. This greatly increases the cost of the interrupter. It also necessitates larger separation between the exterior magnetic field coils used to suppress the arc. Interelectrode field strength is thereby lowered, resulting in reduced performance.